In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode, a polymer sustained alignment (PSA) mode and a field induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The devices include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship of the general characteristics between two aspects. The general characteristics of the composition will be further explained based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is approximately 70° C. or higher and a preferred minimum temperature of the nematic phase is approximately −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred. An elastic constant in the composition relates to contrast of the device. A larger elastic constant in the composition is further preferred to increase the contrast in the device.
TABLE 1General Characteristics of Composition and AM DeviceNo.General Characteristics of CompositionGeneral Characteristics of AM Device1Wide temperature range of a nematic phaseWide usable temperature range2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negative dielectricLow threshold voltage andanisotropysmall electric power consumptionLarge contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to ultraviolet light and heatLong service life7Large elastic constantLarge contrast ratio and shortresponse time1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of the composition relates to a contrast ratio in the device. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is in the range of approximately 0.45 micrometer in a device having the mode such as the TN mode. The suitable value is in the range of approximately 0.30 micrometer to approximately 0.40 micrometer in a device having the VA mode, and in the range of approximately 0.20 micrometer to approximately 0.30 micrometer in a device having the IPS mode or the FFS mode. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large absolute value of dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large absolute value of dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio, and contributes to a large contrast ratio in the device. Accordingly, a composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device used in a liquid crystal projector, a liquid crystal television and so forth. A large elastic constant in the composition contributes to a large contrast ratio and a short response time in the device. Accordingly, the large elastic constant is preferred.
As described above, the liquid crystal composition is required to have a large voltage holding ratio, and as a result, use of a liquid crystal compound having a large specific resistance is required. On the other hand, even with the thus carefully prepared liquid crystal composition, generation of a poor display by a change over time is known. The generation is considered to be caused by formation of a highly polar substance by a liquid crystal composition being gradually oxidized by action of heat, light or air under use conditions, and thus an instance of a decrease in a voltage holding ratio is considered to be one of the causes of the poor display.
Then, methods for adding a known antioxidant such as 2,6-di-tert-butyl-4-methylphenol (BHT) in order to prevent oxidization of the liquid crystal compound are disclosed in Patent literature Nos. 1 and 2. However, the antioxidants are known to cause, owing to a high vapor pressure, insufficient compatibility with the liquid crystal compound or the like, a problem of incapability of obtaining a desired effect by a decrease in an amount of the antioxidant in the liquid crystal composition, or a problem of causing precipitation of a crystal under low temperature conditions.
In order to solve the problems, an attempt for adding a new antioxidant has been made. For example, Patent literature No. 3 describes addition of a compound represented by formula (S-1) below to a liquid crystal composition, and Patent literature No. 4 describes addition of a compound represented by formula (S-2) below to a liquid crystal composition. However, the compound has a 2,6-di-tert-butylphenol moiety, and is required to be added in a high concentration in order to obtain an expected effect. Moreover, the compound has a poor compatibility with a liquid crystal composition to cause a poor display under low temperature conditions. Non-patent literature No. 1 describes antioxidant activity of phenol in which bulkiness on 2-position or 6-position is decreased. However, the relevant literature refers to nothing for stabilization of the liquid crystal composition.

A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode or the FFS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the PSA mode or the FPA mode. Examples of liquid crystal compositions each using an antioxidant are disclosed in Patent literature Nos. 1 to 4.